Precise Characterization of Resists and Thin Gate Dielectrics in the VUV Range for 157nm Lithography
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Precise Characterization of Resists and Thin Gate Dielectrics in the VUV Range for 157nm Lithography Pierre Boher*, Patrick Evrard, Jean Philippe Piel, Christophe Defranoux and Jean Louis Stehlé, SOPRA S.A., 26 rue Pierre Joigneaux, 92270 Bois Colombes, France. ABSTRACT Spectroscopic ellipsometry is one of the most important tools for thin film metrology. It is now intensively used in microelectronics and especially for the microlithographic applications. Instrumentation for the next generation of VUV lithography at 157nm requires special optical setup since oxygen and water are extremely absorbing below 190nm. Recently a new ellipsometer included in a purged glove box to reduce the oxygen and water contamination in the part per million range has been developed. In the VUV range, roughness and interface diffusion become critical since the layer thickness is generally reduced. An independent characterization technique like the grazing x-ray reflectance is then extremely complementary to ellipsometry in this wavelength region. The present paper presents recent results on up to date lithography samples combining the two characterization techniques. Photoresists and gate dielectrics are successively examined. INTRODUCTION Recently, there has been increasing interest in using 157nm F2 laser sources in projection lithography as successors to 193nm based systems. This would follow the historical trends in the microelectronic industry where wavelength reduction is used to improve feature resolution. At 248 and 193nm, spectroscopic ellipsometry (SE) has proved to be a very efficient method to characterize photoresists and antireflective coatings1. Compared to more simple techniques like reflectance, ellipsometry has several advantages. First, the measurement is made on a ratio of two signals (Rp/Rs where Rp and Rs are the reflection coefficients of the two polarisations parallel and perpendicular to the incidence plane). So the measurement is less dependent on the source fluctuations and the accuracy of the measurement is generally better than a photometric one. There is also no need of a reference sample since the measurement is self-calibrated. Finally, two independent parameters (amplitude ratio and phase) are measured simultaneously instead of one for reflectance or transmittance, which allows direct extraction of complex indices without using Kramers-Kronig methods. The measurement can also be rapid thanks to the development of multichannel detectors2. Photoresist behavior versus exposure dose has for example been determined by this method3. At 157nm, the layer thickness is generally smaller than for the current lithographic generation. So the correlation between thickness and indices is enhanced making more difficult the use of photometric techniques. Grazing x-ray reflectance (GXR) is also a technique of choice for measurement of thin film thickness without need of a structural model4-5 as n is always close to 1 and k negligible at this wavelength (1.54 Å). In the case of multilayers, precise information can also be e
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